The aims of this proposal cover a variety of biochemical and molecular approaches to determine the strain-specific differences between resistant (R) vs. susceptible (S) B. glabratathat would lead to a greater understanding of defense activity in the snail. The PI hypothesizes that: (a) there can be strain-specific differences in the respiratory burst of hemocytes; (b) that sensitivity differences exist in the S and R hemocytes to negative influence of a normal intermediate of the respiratory burst (e.g. chlorinated amines) and; (c) there may be strain differences in induction of specific gene transcripts. Much of the work in this proposal centers on the use of an in vitro assay system for determining sporocyst killing, and the PI has developed ways for selectively inhibiting certain enzymes in O2 and N dependent pathways. Since the last renewal the PI lists 11 manuscripts (published and in press/submitted) and 8 review articles dealing with various aspects of this snail/trematode relationship. Several other papers not directly related are listed that serve as support for demonstrating the PI's expertise in techniques for future studies. Findings during the last funding period led to the understanding that, at least in vitro, both O2 - dependent and -independent mechanisms may play a role in sporocyst killing. This came about after the CMC assay was substantially improved, and finding that H2O2 was involved in killing (although they failed to show differences in ROS production in R and S hemocytes in non-CMC assays). These experiments led to the premise that destruction of sporocysts in resistant snails may be due to a defective myeloperoxidase (MPO) that allows H2O2 to mediate killing, whereas MO hemocytes detoxify H2O2 before it is able to exert its effect fully. The PI also has developed substantial evidence that NO and H2O2 play synergistic roles in sporocyst killing. These results will serve as a basis to explore further these two pathways in sporocyst killing.